Preliminary investigations in animals and man with acute myocardial infarction suggest that coronary vasodilators may improve collateral blood flow and reduce ischemic injury under certain conditions, but cause decreased flow under other conditions ("coronary steal"). Conditions of coronary anatomy, hemodynamics, and infarction size vary markedly in patients with acute infarction. It is therefore imperative to understand the mechanisms of vasodilator effect to rationally and effectively use these drugs in the complexity of the clinical setting. The effect of different types of coronary dilators (large vessel dilators like nitroglycerin, small vessel dilators like dipyridamole, combined dilators like nitro-prusside and nifedipine, and the specific coronary arteriolar dilator (Abbott 40557) will be determined in anesthetized and conscious dogs and the isolated, isovolumic, blood-perfused canine heart after coronary artery ligation. Myocardial blood flow to ischemic and non-ischemic regions will be determined with radioactive microspheres, ischemic injury by ST segment elevation in epicardial electrograms, and regional myocardial function by sonomicrometer. Dogs will be studied allowing heart rate, blood pressure, and ventricular size to change, under conditions when these parameters are controlled, and during autonomic ganglionic coronary artery stenosis. Intracoronary pressure measurements will be made and correlated with changes in collateral flow. Pathologic infarct size will be measured in the chronic dog studies. In the isolated heart experiments, the importance of direct effects of vasodilators (on vascular tone) will be compared to indirect effects (on hemodynamics, leading to changes in myocardial wall tension).